When replacing the wishbones on my MK4 golf, the front most captive nut for the bolt that goes through the bush has come loose/un tacked

I've been advised that this will require a new subframe, however, is it possible to re-weld this on?

If you can get at it you should be able to weld it. You can make a plate with a nut welded to it and weld it in place if you can make a hole for it to go in.